The fabrication of electronic microcircuits requires the use of substrates, heatsinks, electrodes, leads, connectors, packaging structures and other components capable of dissipating the heat generated by the active parts of the microcircuit or by the soldering, brazing or glass-sealing process. Moreover, those components that are in direct contact with the active microcircuit sections must have a coefficient of thermal expansion compatible with gallium arsenide, polysilicon, germanium or any material used in the fabrication of the microcircuit.
Materials such as copper, silver, gold and aluminum which exhibit high coefficient of heat dissipation tends also to have coefficients of expansion much higher than materials such as gallium arsenide alumina or polysilicon which are used in the implementation microcircuit elements or their enclosures.
As disclosed in U.S. Pat. No. 4,680,618 Kuroda et al., it has been found convenient to use composites of copper and other denser metals such as tungsten or molybdenum in the fabrication of heatsinks, substrates and other heat-dissipating elements of microcircuits. The proportions of the metals in the composite are designed to match the coefficient of thermal expansion of the material used in the fabrication of the active circuit component.
The coefficient of thermal expansion of a metal is defined as the ratio of the change in length per degree Celsius to the length at 0.degree. C. It is usually given as an average value over a range of temperatures. Metal used in electrical conductors such as aluminum, copper, silver and gold that have a low electrical resistivity also exhibit high coefficients of thermal conductivity. The coefficient K of thermal conductivity of a material is defined as the time rate of heat transfer through unit thickness, across unit area, for a unit difference in temperature or K=WL/A.DELTA.T where W=watts, L=thickness in meters, A=area in square meters, and T=temperature difference in .degree.K. or .degree.C. For copper, K is equal to 388. For silver, K is equal to 419. However, these metals exhibit an average coefficient of thermal expansion in excess of 15.times.10.sup.-6 /.degree.C. By contrast, material such as gallium arsenide and silicon, that are commonly used in the manufacture of microcircuit chips have an average coefficient of thermal expansion of less than 7.times.10.sup.-6 /.degree.C. Thus, while material of high electrical and thermal conductivity are favored in the fabrication of heat-dissipating electric elements, they must be blended with conductive material exhibiting a much lower average coefficient of expansion in order to create a composite whose thermal expansion characteristic comes as close as practically possible to that of gallium arsenide, silicon and other micro-chip materials. Tungsten and molybdenum with average coefficient of thermal expansion of 4.6.times.10.sup.-6 /.degree.C. and 6.times.10.sup.-6 /.degree.C. and coefficient of thermal conductivity of 160 and 146 respectively are favored.
However, while copper, aluminum, and silver have specific gravities of less than 9, and melting point of less than 1,100 .degree.C., tungsten and molybdenum have specific gravities of 19.3 and 10.2, and melting points of 3,370.degree. C. and 2,630.degree. C. respectively.
Due to the large differences in the specific gravities and melting-points, and lack of mutual solubility of metals such as copper and tungsten, for example, it is difficult to form composites of those two metals that exhibit a reliable degree of homogeneity using conventional melting processes.
As disclosed in U.S. Pat. No. 5,086,333, it has been found more practical to press and sinter a powder of the most dense materials, e.g., tungsten, to form a porous compact, then to infiltrate the compact with molten copper or another lighter material. A slab of the resulting material can then be cut and machined to form heatsinks, connectors, substrates and other heat-dissipating elements.
The instant invention results from an attempt to devise a simpler and more practical process to manufacture such heat-dissipating components using powder metallurgy techniques.